C&G-27-pzui最小流化速度

NOX EXPERIMENTAL STUDY ON RDF COMBUSTION IN A

FLUIDIZED BED

Zhao Song1，Yan Chang-feng1,2，Li Hai-bin1，Zhao Zeng-li1

1. Guangzhou Institute of Energy Conversion, CAS, Guangzhou, China, 5100

2. University of Science and Technology of China, Hefei, China, 230027

Abstract：Fluidization behaviors of particles and NOx formation from combustion of two kinds of Refuse Derived Fuel (RDF), which containing different content of nitrogen element are fired in a fluidized bed, were studied. It shows that NOx formation during combustion changes with temperature in combustion chamber; both excess air coefficient and the injection of secondary air may influence NOx production; the more nitrogen element that RDF contains, the higher NOx concentration in flue gas; and NOx formation from the RDF with more nitrogen element is easier to be influenced by the change of other operating conditions.

Key words：Refuse derived fuel; fluidized bed; NOx; combustion

1. Introduction

In recent years, the negative effect on environmental pollution brought by the rapid development of the economy and urbanization becomes more noticeable in China. According to the statistics, the total amount of the Municipal Solid Waste (MSW) yielded in china has been up to1.9×108 tons in 2003, and annually increases at the rate of 8~10%. MSW without sanitary landfill can pollute soil and groundwater, especially when MSW is incinerated, it will produce caustic gases, such as NOx，SOx and HCl [1-4], which may result in acid rain. With the environment problem being paid more attention to, it has become very urgent to be solved.

MSW is also a kind of resource to some extent. The combustible components of dried MSW mixing with de-chlorination additives are crushed and shaped into RDF with uniform size, high heating value and easier transportation [1,5]. It will be one of the promising technical means to exploit energy efficiently from MSW in the future. RDF has been industrialized and commercialized in some developed countries. At present, energy resource has been thought as a strategic issue because of serious deficiency in china, so RDF is very hopeful to become a very important potential energy resource. In addition, tax on NOx emission from the oil or coal-fired power plants has been levied some provinces (i.e. Guangdong province) in China. Therefore, NOx formation from the combustion of two kinds of RDFs in the fluidized bed was investigated, which offers certain theoretical and experimental foundations for its practical utilization.

2.Experiments

2.1 The preparation of RDFs and silica sandThere mainly are six kinds of combustible materials in MSW, such as plastic, waste paper, grass, wood, fabric, rubber, and leather. Plastic and waste paper account for 60%~70% in the MSW except food residue[6]. Waste paper and granular plastic was mixed, heated and shaped into

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cylindric RDF with 6 mm diameter and 20-30 mm long by mold machine. The approximate and ultimate analysis of two kinds of raw materials used in this experiment is listed as Table 1.

Table 1. Approximate and ultimate analysis of RDFs

Material

V

FC

A

C H O 70.0910.92

18.9

N S HHV (kJ/kg) 0.040.06

27668.7 33343.4

RDF-a 95.18 3.62 1.2RDF-b 86.78 7.13 6.09

59 18.9720.061.820.15

The fluidized-bed bed material is silica sand whose properties of particle are shown in Table

2. The particle size of sand 1 is mainly ranging between 0.05~0.16 mm and 0.28~0.335 mm; sand 2 has a relatively wide particle size distribution, majority of which is larger than 0.22 mm; the particle size of sand 3 mostly is larger than 1 mm. According to Geldart sorting method, sand 1 belongs to particle of medium granularity and has good fluidizability. In addition, the Gaussian distribution of the granularity is helpful to be fluidized, so sand 1 is easier to be fluidized transformed form the bubbling immediately as soon as the superficial air velocity reaches the critical fluidizing velocity[7,8]. Sand 2 and 3 belong to the bed material that is relatively difficult to be fluidized and the superficial air velocity in the experiment has not reached its minimum fluidized velocity, which is in accordance with the reference.

Table2. Characteristic of silica sand

Particle characteristic, mm 1 2 3

>1.000 0.3414.77 98.93 0.500-1.000 0.8611.30

0.68 0.355-0.500 16.1850.58 0.280-0.355 0.290.220-0.280 0.740.35 Diameter,

0.160-0.220 0.003.31

%

0.100-0.160 44.9511.18

0.061-0.100 3.68 28.48

0.04 0.050-0.061

4.42 0.040-0.050 0.07>0.040 8.16Average diameter, µm 85 304 1447 True density, kg/m324752592 25 Stacking density, kg/m313611338 1373

2.2 Experimental apparatusThe fluidized-bed unit with tube-type air distributor is shown in Figure 1. The combustion chamber with the inner liner of anticorrosive refractory is divided into three parts with different cross section area. The lower part of the combustion chamber with a cross section area of 300×300 mm2 and height of 3050 mm is mainly for the gasification of material; the upper part,

5

600×600 mm2 and 990 mm, increases the residence time of the material in the combustion chamber and thus improves the combustion efficiency; the mid-part has a changing cross section area. Four pipes of 20 mm diameter as air distributor, marked as A, B, C, and D respectively, are uniformly distributed at the bottom of the combustion chamber. Each of the pipes has two rows of holes of 3 mm diameter along the axis. The pipes are divided into two groups (I(A,D), II(B,C)) based on the angle of 90° and 135° respectively between rows of holes.. The positions of testing temperature over air distribution tube are 41 cm、94 cm、155 cm、204 cm、2 cm、294 cm、3 cm、400 cm along height respectively.

The material is supplied by automatic conveying system. Through a self-adjusting door, the material in the hopper flows onto the conveying belt that can weigh the material automatically. Then the material is conveyed into the pneumatic feeding hole and the feed speed is adjusted by the pneumatic frequency.

13915101113122876144

1.conveying belt 2.pneumatic valve 3.combustion chamber 4.air blower 5. air-preheater 6.fluidized air 7.primary air 8.secondary air 9.cyclone 10.jacket of water cooling pipe 11.bag filter 12.draft fan 13.chimney 14.inlet of cooling water 15.outlet of cooling water

Figure 1. The schematic diagram of fluidized bed RDF combustion system

2.3 Experiment methodsSilica sand was added into the bed as fluidized material. It was pre-heated by high temperature flue gas produced from an oil-fired burner. When the temperature of sand was up to 200 , ℃the charcoal ignited partly was added to the bed and the burner was shut off. The bed temperature was up to 600 ℃ after the charcoal combusted sufficiently and then RDF was put into the bed. When the system operated stably, the flue gas was sampled and analyzed.

3. Results and discussions

3.1 Fluidization of particle in the fluidized bedBase on the results of drag force of the orifices on the single tube-type(I), it shows that their pressure drop is proportional to the square of the air velocity as shown in Figure 2. If the orifice

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symmetrically opens downwards, the kinetic energy of the air is lost during the process of the flowing downwards. The smaller the angle of the orifices opened is, the more the kinetic energy is lost. The drag force of the orifices opened downwards is bigger than that of the opened[9] upwards ones. With the increase of the superficial air velocity through the orifices, the drag force increases faster as shown in Figure 3.

2000

2000

Pressure loss, Pa1000

Pressure loss, Pa1500

1500

I II ZheJiang University1000

500500

00

51015202530355101520253035

Figure 2.Drag force of the orifices on Figure 3. Drag force of the orifices the single tube-type on the tube-type distributor

Velocity of air, m/s

600060005500Velocity of air, m/s

500050004500Pressure difference, PaPressure difference, Pa40004000350030002500200015001000500 sand 1 100 mm 200 mm 300 mm 400 mm 500 mm3000Height of bed materail (500 mm) sand 1 sand 2 sand 32000100000.000.030.060.090.120.1500.000.030.060.090.120.150.180.21Superficial velocity, m/s

Figure 4. Fluidization properties of different Figure 5.Effect of bed material height on bed materials fluidization

Only sand 1 may be fluidized under experimental conditions as shown in Figure 4. After the superficial air velocity of sand 1 is more than the critical fluidizing velocity of 0.045 m/s, it starts to fluidized. The pressure drop of sand 2 and sand 3 increases with the superficial air velocity. Because the size and the density of sand 2 and sand 3 are comparatively bigger, sand can’t be fluidized under the operational conditions. The height of material 1 increases, but the starting fluidizing velocity is changed little as shown in Figure 5. As the general fluidization, the starting fluidizing velocity of the bed material is only related to the properties of the bed material, not to the height of bed material.

3.2 NOx formation During combustion processes, nitrogen oxides can be formed from molecular nitrogen in the

Superficial velocity, m/s

591

combustion air or from nitrogen chemically bound in the fuel. NOx formation can be classified as thermal NOx, fuel NOx, prompt NOx. Thermal NOx is mainly produced above 1800 K [1]. So combustion product NOx in a fluidized bed is primarily fuel NOx and prompt NOx.

NOx concentration and other gas component are listed in Table 3., When the total air supply keeps the same, increase of secondary air ratio can reduce CO and CxHy concentration levels that lead to decrease of prompt NOx from combustion of RDF-a. In addition, fuel NOx concentration is limited because of little content of N element in RDF-a. Therefore, the injection of secondary air can change NOx concentration in flue gas from RDF-a.

Table.3 NOx concentration and other gas component from combustion of RDF-a SN CO, % CxHy, % O2, % CO2, %1 2 3 4

3.01 1.38 0. 1.31

2.26 0.27 0.14 0.12

5.34 4.7 4.83 5.22

9.76 9.96 11.77 11.36

Primary air, m3/h

55 50 48 46

Secondary air, m3/h NOx, mg/m3

0 5 7 9

.5 88.8 84.7 82.7

As shown in Figure 6, the temperature and NOx concentration vary with the heights in the chamber. The temperature in the chamber is up to the highest at the inlet of secondary air and then decreases with the height. RDFs are pyrolyzed and gasified through absorbing heat from the fluidized material and products from pyrolysis and gasification start to be fired after secondary air is supplied. The distribution of NOx concentration is similar to that of the temperate along the height.

1000900800700

200

Temperature NOx200

180

180

160140120

RDF-a RDF-b600500400300

05010015020025030035040010080

NOx(mg/m3)16014012010080

NOx(mg/m3)T(℃)60450 60

H(cm)

800850

T(℃)

9009501000

Figure 6. Temperature and NOx concentration Figure 7. Variation of NOx concentration distribution with temperature of flue gas

(feed rate=5 kg/h, Qf=70 m3/h, RDF-b) (feed rate=5 kg/h, Qf=70 m3/h)

NOx products mainly include NO accounting for majority of NOx, and NO2, which is formed in downstream flue gas or transformed from some of NO. As shown in Figure 7., NOx concentration slightly varies with temperature when nitrogen content is 0.04% for RDF-a, but

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NOx concentration from RDF-b significantly increases with temperature. It suggests that the effect of different nitrogen-contained fuels on content of the fuel NOx in total NOx is remarkably different. NOx concentration drops off with increase of excess air coefficient under operational conditions as shown in Figure 8.

1801601401201008060

180

RDF-a RDF-b RDF-a RDF-b160

NOx(mg/m3)NOx(mg/m3)140

120

100

80

0.81.01.2

Figure 8. Effect of excess air ratio on NOx Figure 9. Effect of injection of secondary air concentration in flue gas on NOx concentration

RDF-a (total air=55 m3/h) RDF-b (bed air=70 m3/h)

The effect of injection of secondary air on NOx concentration as shown in Figure 9., NOx concentration slightly changes as total air supply for RDF-a maintains 55 m3/h, but it decreases dramatically with the increase of injection of secondary air for RDF-b when the primary air maintains 70 m3/h. It suggests that the effect of different nitrogen-contained fuels on content of the fuel NOx is remarkably important.

excess air ratio

1.41.61.82.02.22.42.6-101

secondary air injection(m3/h)

2345671011

4.Conclusion

Combustion of two kinds of RDFs containing different nitrogen element was studied in a fluidized bed. Conclusions can be generated as the following:

1． RDF material can be fired completely in a fluidized bed, and the NOx production

increases with the temperature increase in the fluidized bed on the whole.

2． Under the same operating conditions, NOx product during combustion of RDF with

more nitrogen element is significantly more than that of RDF with less nitrogen element. 3． An increase of the excess air coefficient leads to a decrease of NOx concentration under

the operational condition.

4． Under the same operating conditions, the two-staged combustion decreases NOx

production, and it is more effective in reducing NOx level for the RDF with more nitrogen element.

Acknowledgements

Support for this work was provided by National Natural Science Foundation of China.

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References

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